The present invention relates to the field of image intensifying devices using solid-state sensors, such as a CMOS or CCD device. Image intensifier devices are used to amplify low intensity light or convert non-visible light into readily viewable images. Image intensifier devices are particularly useful for providing images from infrared light and have many industrial and military applications. For example, image intensifier tubes are used for enhancing the night vision of aviators, for photographing astronomical bodies and for providing night vision to sufferers of retinitis pigmentosa (night blindness).
There are three types of known image intensifying devices in prior art; image intensifier tubes for cameras, all solid-state CMOS and CCD sensors, and hybrid EBCCD/CMOS (Electronic Bombarded CCD or CMOS sensor).
Image intensifier tubes are well known and used throughout many industries. Referring to FIG. 1, a current state of the prior art Generation III (GEN III) image intensifier tube 10 is shown. Examples of the use of such a GEN III image intensifier tube in the prior art are exemplified in U.S. Pat. No. 5,029,963 to Naselli, et al., entitled REPLACEMENT DEVICE FOR A DRIVER'S VIEWER and U.S. Pat. No. 5,084,780 to Phillips, entitled TELESCOPIC SIGHT FOR DAYLIGHT VIEWING. The GEN III image intensifier tube 10 shown, and in both cited references, is of the type currently manufactured by ITT Corporation, the assignee herein. In the intensifier tube 10 shown in FIG. 1, infrared energy impinges upon a photo cathode 12. The photo cathode 12 is comprised of a glass faceplate 14 coated on one side with an antireflection layer 16, a gallium aluminum arsenide (GaAIAs) window layer 17 and a gallium arsenide (GaAs) active layer 18. Infrared energy is absorbed in GaAs active layer 18 thereby resulting in the generation of electron/hole pairs. The produced electrons are then emitted into the vacuum housing 22 through a negative electron affinity (NEA) coating 20 present on the GaAs active layer 18.
A microchannel plate (MCP) 24 is positioned within the vacuum housing 22, adjacent the NEA coating 20 of the photo cathode 12. Conventionally, the MCP 24 is made of glass having a conductive input surface 26 and a conductive output surface 28. Once electrons exit the photo cathode 12, the electrons are accelerated toward the input surface 26 of the MCP 24 by a difference in potential between the input surface 26 and the photo cathode 12 of approximately 300 to 900 volts. As the electrons bombard the input surface 26 of the MCP 24, secondary electrons are generated within the MCP 24. The MCP 24 may generate several hundred electrons for each electron entering the input surface 26. The MCP 24 is subjected to a difference in potential between the input surface 26 and the output surface 28, which is typically about 1100 volts, whereby the potential difference enables electron multiplication.
As the multiplied electrons exit the MCP 24, the electrons are accelerated through the vacuum housing 22 toward the phosphor screen 30 by the difference in potential between the phosphor screen 30 and the output surface 28 of approximately 4200 volts. As the electrons impinge upon the phosphor screen 30, many photons are produced per electron. The photons create the output image for the image intensifier tube 10 on the output surface 28 of the optical inverter element 31.
Image intensifiers such as those illustrated in FIG. 1 have advantages over other forms of image intensifiers. First, intensifiers have a logarithmic gain curve. That is, the gain decreases as the input light level is increased. This matches the human eye response particularly when bright lights are in the same scene as low lights. Most solid-state devices have a linear response; i.e., the brighter the light the brighter the output signal. The result is that bright lights appear much brighter to a viewer of a solid-state system and tend to wash out the scene. Solid-state sensors can be modified to produce a gain decrease as input light is increased, however, this requires changing the amplifier gain, using shuttering, or using anti-blooming control.
Another advantage of image intensifiers is the ability to function over a large range of input light levels. The power supply can control the cathode voltage and thereby change the tube gain to fit the scene. Thus tubes can function from overcast starlight to daytime conditions.
However, image intensifier/I2 cameras suffer from numerous disadvantages. The electron optics of the phosphor screen produces a low contrast image. This results in the object looking fuzzier to the human observer, or solid-state sensor, when viewed through an image intensifier. Although this deficiency has been somewhat reduced with further image intensifier development, solid-state imagers generally have better performance.
Another disadvantage with image intensifier/I2 cameras is “halo.” Halo results from electrons being reflected off either the MCP or the screen. The reflected electrons are then amplified and converted into light in the form of a ring around the original image. In image tubes, the halo from electrons reflected from the MCP has been reduced to a negligible effect for the most recent production tubes. However, the halo from the screen section still exists, although not to the degree of the cathode halo. Nevertheless, the screen halo is still a significant defect in imaging systems when a CCD or CMOS array is coupled to the image intensifier. This is because these arrays are more sensitive than the eye to the low light levels in the screen halo.
Another disadvantage is that image intensifiers do not have a method of providing electronic read-out. Electronic read-out is desired so that imagery from thermal sensors may be combined with intensified imagery with the result that the information from both spectra will be viewed at the same time. One solution has been to create an I2 camera by coupling a CCD or CMOS array to an image intensifier tube. When a solid-state device is coupled to an image tube the resultant camera has all performance defects of the image tube that is low contrast, often poor limiting resolution due to coupling inefficiencies and the added cost of the image tube to the camera.
Solid-state devices typically include CCD or CMOS. They function by directly detecting the light, electronically transferring the signal to solid-state amplifiers, then displaying the image on either a television type tube or display such as a liquid crystal display. FIGS. 2a and 2b illustrate a flow chart and schematic diagram for a typical CCD sensor.
CCD and CMOS sensors are solid-state devices; that is, there is no vacuum envelope and the output is an electronic signal that must be displayed elsewhere and not within the sensor. The solid-state devices operate with power of 5–15 volts. The light is detected in individual pixels as labeled “s” and translated into electrons that are stored in the pixel until the pixel is read out to the storage register. From the storage register the electronic information contained in multiple pixels is then transferred to a read out register and then to output amplifiers and then to a video display device such as a cathode ray tube.
The disadvantages of an all solid-state device are poor low light level performance, potential blooming from bright light sources, poor limiting resolution, and high power consumption. The poor low light performance is due to dark current and read-out noise resulting in low signal-noise ratios. If a signal gain mechanism were provided prior to read-out this issue would be negated, as sufficient signal would exist to overcome the noise sources. Solid-state device architectures usually do not permit an amplification section prior to read-out. The poor limiting resolution is due to large pixel sizes usually chosen in an attempt to collect a large signal and thereby increase the signal to noise ratio. These disadvantages have effectively prevented the use of solid-state sensors in night vision applications. The advantages of solid-state devices are better image contrast as compared to the image intensifier/I2 camera, the availability of electronic read-out, and lower cost, particularly when the solid-state sensor is a CMOS array.
As can be seen, the strengths and weaknesses of image intensifiers and solid-state sensors compliment each other and theoretically a combination of both devices would give better performance. One such combination proposed as an alternative to image intensifiers/I2 cameras and solid-state sensors, is the electron bombarded CCD/CMOS sensor (EBCCD/CMOS). This device consists of the photo-cathode and body envelope of the image tube, and either a CCD or CMOS sensor integrated into this envelope. An illustrative example of an EBCCD/CMOS sensor is shown in FIG. 3. A high voltage is applied between the cathode and solid-state sensor so that the resulting electrons are amplified within the silicon in the solid-state sensor by electron bombardment.
The advantages of the EBCCD/CMOS device are that it provides electronic read-out. But the disadvantages are numerous. First, the intra-scene dynamic range is compressed. This means that overall contrast within the scene, when bright objects are next to dark objects, is reduced compared to an image intensifier/I2 camera and all solid-state device. Secondly, the sensor suffers “halo” degradation of the image around bright lights due to electrons reflected off of the solid-state sensor. This halo exists in regular image tubes; however, technological improvements have reduced the halo to the point of non-existence. Thirdly, the very high voltage required to operate the device (2–10 kV) damages the silicon surface causing decay in performance over time.
Therefore, it is an object of the present invention to provide an intensified hybrid solid-state sensor that combines the functions of the image intensifier, good signal-to-noise ratio and high logarithmic gain, with the electronic read-out functions either of a Complementary Metal Oxide Semiconductor (CMOS) or Charged Coupled Device (CCD).